1. Field of the Invention
The present invention relates to a network relay device, and in particular to, a network relay device handling contents information encrypted by DTCP (Digital Transmission Content Protection) standard, a communication device and a network relay method.
2. Description of the Related Art
Recently, in accordance with development and wide application of digital equipment, demands for such network communication devices have been growing. In such network communication devices, high-speed and multi-functional data transmission is performed by using digital transmission.
For example, in accordance with digital transmission, data (contents) flowing on the transmission is easily tampered and copied and thus techniques for protecting the contents are required. One of the techniques standardized for such protection is DTCP standard (see http://www.dtcp.com). The technique is developed in order to protect digital synchronous packet data flowing on an IEEE1394 high-speed serial bus against tampering and illegal copying. The specification is disclosed on the aforementioned URL, and is used for passing data with digital televisions or digital VHSs.
The DTCP standard is to be extended to a standard for, as well as the IEEE1394 high-speed serial bus, the case of transmitting on other bus standards such as USB (Universal Serial Bus) or the case of transmitting over different networks.
In accordance with prior arts using the DTCP standard, a network communication device and a network relay device perform encryption and decryption processings based on the DTCP standard and network communication while a security against the third party being maintained. In accordance such prior arts, however, a processing for researching key information used for encryption is not performed.
In accordance with the DTCP standard, when a sending communication device stops sending of contents and then resumes, the sending communication device may encrypt the contents with a cryptographic key which is different from the cryptographic key used before resumption and send the encrypted contents. A receiving communication device can detect that transmission of the contents is stopped but cannot strictly and correctly make a determination. For this reason, the receiving communication device must confirm, every time when the sending is stopped, a key number (key number which is changed every time when the contents is sent) by using an AKE command.
Accordingly, when sending of the contents encrypted based on the DTCP standard from the sending communication device is stopped, the network relay device also stops the contents to be sent to the receiving communication device. The receiving communication device confirms a key number by using an AKE command even when, as well as the key number is changed, sending is delayed simply because of noises. Thus, there arises a problem about a delayed sending/receiving speed.
In accordance with conventional network relay devices, when communication for encrypted contents based on the DTCP standard is performed, if the contents from a sending device is delayed, a key may be changed. For this reason, a network relay device temporarily stops sending of the contents to a receiving network relay device and must inspire a receiving communication device to confirm a key number by an AKE command. Nevertheless, the key number is not necessarily changed and the contents information may be delayed simply because of noises. As a result, the receiving communication device repeats waste key number confirmation, resulting in a delayed communication.